Manhole structures are typically cast at the factory using the mold forms appropriate for forming each section. Manhole bases are cast using concentric inner and outer mold forms which define the walls of the manhole base. The manhole base is cast "upside down". The top member of the inner mold is typically provided with raised projections which define the invert to be formed in the manhole base. Since a variety of inverts can be formed, such as, for example, a straight-through invert, an acute angle invert, a right angle invert, and a Y-shaped invert, to name just a few possible arrangements, a separate top section is required for each invert configuration. One technique for forming a monolithic manhole base in a single pour operation, is described in copending application Ser. No. 234,639 filed Feb. 17, 1981 by this inventor, now U.S. Pat. No. 4,422,994 issued Dec. 27, 1983, and comprises a fixed invert forming section and a movable invert forming section the inner ends thereof joined by a flexible "knee" portion. However, the apparatus of the aforementioned copending application lacks means for simply and positively locating the movable invert forming section and is further incapable of forming three branched inverts, such as a Y-shaped invert. In addition, the flexible joining portion of the aformentioned copending application will form a central curvature which departs from the desired curvature.